NEW CHEMICAL REACTIONS. 83 



ness, forming particles distinguishable by the naked eye, 

 or far beyond the reach of our highest microscopic powers. 

 I have no reason to doubt that particles may be thus 

 obtained, whose diameters constitute but a small fraction 

 of the length of a wave of violet light. 



In all cases when the v-apors of the liquids employed are 

 sufficiently attenuated, no matter what the liquid may be, 

 the visible action commences with the formation of a blue 

 cloud. But here I must guard myself against all miscon- 

 ception as to the use of this term. The " cloud " here 

 referred to is totally invisible in ordinary daylight. To 

 be seen, it requires to be surrounded by darkness, it only 

 being illuminated by a powerful beam of light. This blue 

 cloud differs in many important particulars from the finest 

 ordinary clouds, and might justly have assigned to it an 

 intermediate position between such clouds and true vapor. 

 With this explanation, the term "cloud," or "incipient 

 cloud, "or "actinic cloud/ 7 as I propose to employ it, can- 

 not, I think, be misunderstood. 



I had been endeavoring to decompose carbonic acid gas 

 by light. A faint bluish cloud, due it may be, or it may 

 not be, to the residue of some vapor previously employed, 

 was formed in the experimental tube. On looking across 

 this cloud through a NicoPs prism, the line of vision being 

 horizontal, it was found that when the short diagonal of 

 tile prism was vertical, the quantity of light reaching the 

 eye was greater than when the long diagonal was vertical. 

 When a plate of tourmaline was held between the eye and 

 the bluish cloud, the quantity of light reaching the eye 

 when the axis of the prism was perpendicular to the axis 

 of the illuminating beam, was greater than when the 

 six is of the crystal and of the beam were parallel to each 

 other. 



This was the result ail round the experimental tube. 

 Causing the crystal of tourmaline to revolve round the tube, 

 with its axis perpendicular to the illuminating beam, the 

 quantity of light that reached the eye was in all its positions 

 a maximum. When the crystallographic axis was parallel 

 to the axis of the beam, the quantity of light transmitted 

 by the crystal was a minimum. From the illuminated 

 bluish cloud, therefore, polarized light was discharged, the 

 direction of maximum polarization being at right angles to 



